1. Technical Field
The present invention relates to a light-emitting device, and more particularly, to a light-emitting device, which illuminates a gas with microwaves.
2. Related Art
An Industry Science Medical band (ISM band) using microwaves is applied to various devices such as light-emitting devices, heating devices, plasma generators, communication devices and radar units. In one of these devices, a magnetron is used as an oscillation source to generate microwaves.
JP-A-H9-265914 discloses a magnetron device provided with a high-voltage noise filter. In the disclosure, an insulating layer and a conductive layer on a surface of a coil-shaped conductive wire are provided, and high withstand voltage layers between an outer peripheral surface of the insulating layer and the conductive layer are provided in a vicinity region of the opposing ends of the conductive layer. This structure relaxes concentration of an electric field and improves withstand voltage characteristics of the insulating layer. Further, the structure reduces a thickness of the insulating layer, obtaining a small-sized and low-cost high-voltage noise filter.
Further, JP-A-2004-265611 discloses a plasma processing apparatus. This disclosure indicates that a high-frequency generating source used in the plasma processing apparatus is provided with a magnetron and the like.
As a magnetron is large in size, it was unable to reduce the size and weight of the microwave generator using this. Therefore, when the microwave generator is used for a light-emitting device, it was unable to reduce the size and weight of the light-emitting device. Also, there were problems with the magnetron that it requires a large amount of power, has bad frequency-temperature characteristics, output frequency is unstable and the like.
Further, FIG. 9 is a diagram showing a relationship between the frequency and intensity of signals being output from a magnetron. In FIG. 9, a lateral axis shows frequency and a longitudinal axis shows intensity. A magnetron to generate a microwave of which the certain frequency is f2, also outputs microwaves having frequencies around the frequency f2. For example, when 2.45 GHz is necessary as the certain frequency f2, the magnetron also outputs microwaves having other frequencies around 2.45 GHz since the magnetron outputs frequency signals having a bandwidth. Therefore, there was a problem of harming other devices using the ISM band, because of unwanted radiation generating from the magnetron.
Furthermore, in the microwave generator, not only the magnetron, but an LC oscillator and a dielectric oscillator can also be used as an oscillation source, and frequency signals being output from the oscillation source may also be used by converting into high frequency signals by a PLL (phase-locked loop) circuit and a multiplier circuit. However, the LC oscillator and the dielectric oscillator had problems such as having poor frequency-temperature characteristics, output frequency is unstable and frequency varies with respect to each oscillator. Also, the PLL circuit and the multiplier circuit had problems such as unable to miniaturize as a scale of the circuit is too large, require a large amount of power consumption, and take a long time to output the necessary frequency. And the PLL circuit has a problem that it cannot output the necessary frequency if unlocking occurs.